1. Field of the Invention
The present invention relates to generating time in a signal processing system, and more particularly to a system and method for performing time correction in, for example, a communications system.
2. Background of the Related Art
A Global System Mobile (GSM) terminal and other remote terminals display time using a self-contained RTC circuit, rather than using time information transmitted from a base station.
As shown in FIG. 1, conventional GSM terminals includes a real-time clock (RTC) circuit 10 for counting a low-frequency clock signal of 32.768 KHz and generating time information (second, minute and hour), and a CPU 20 for displaying the time information on LCD 30 generated in synchronization with a high-frequency clock signal of 13 MHz. In the related art, the low-frequency clock signal is generated by a crystal oscillator, and a high-frequency clock signal is a system clock signal generated by a voltage-controlled, temperature-compensated crystal oscillator (VCTCXO).
In generating time information, as previously stated the RTC circuit 10 counts the 32.768 KHz low-frequency clock signal output from the crystal oscillator and generates time information (second, minute and hour). Where the frequency of the currently inputted low-frequency clock signal is precisely 32.768 KHz (X=32768 KHz, as used in the following equation), the RTC circuit 10 sets 1 second when 32768 KHz pulses are counted during a pulse duration and 1 hour when 32768×3600 pulses are counted in the same manner. Thus:                                                         1              32768                        ×            32768                    =                      1            ⁢                                                   ⁢            s            ⁢                                                   ⁢            e            ⁢                                                   ⁢            c            ⁢                                                   ⁢            o            ⁢                                                   ⁢            n            ⁢                                                   ⁢            d                          ⁢                                  ⁢                                            1              32768                        ×            32768            ×            3600                    =                      1            ⁢                                                   ⁢            h            ⁢                                                   ⁢            o            ⁢                                                   ⁢            u            ⁢                                                   ⁢            r                                              (        1        )            where 1/32768 is a pulse duration and 32768 is the count number. The CPU 20 receives the time information output from the RTC circuit 10 and displays it on the LCD 30, so that a user can be aware if the accurate time by viewing the GSM terminal.
Generally, the crystal oscillator used for generating the 32.768 KHz low-frequency clock signal operates at lower power in order to keep power consumption low, and is implemented using a comparatively low-priced crystal. Although the crystal oscillator can maintain a precise dock timing within a narrow temperature range, frequency variation becomes great and timing error is increased if the crystal is exposed to temperatures outside of a predetermined range. Therefore, when the frequency of the low-frequency clock signal generated from the crystal oscillator is changed due to an external condition (ie., a temperature or a voltage, etc), the time information (second, minute, hour) generated by the RTC circuit 10 is degraded in its accuracy.
For example, if the frequency (X) of the clock signal becomes greater than 32.768 KHz due to an external condition (X>32768 KHz), a time error Terror generated during 1 hour can be expressed by equation (2) below.                               T          eroor                =                              (                                          1                32768                            -                              1                X                                      )                    ×          32768          ×          3600                                    (        2        )            
Where the time error (Terror) is converted into the duration of the currently input frequency (X), a pulse count difference generated for 1 hour is computed by the following equation (3).                                           T            eroor                                1            X                          =                  3600          ⁢                      (                          X              -              32768                        )                                              (        3        )            
With reference to equation (3), if the frequency (X) of the low-frequency clock signal is greater than 32.768 KHz, as many as 3600 (X−32768) pulses are additionally counted in the RTC circuit 10 for 1 hour. Thus, the time information outputted from the RTC circuit 10 becomes fast.
If the frequency (X) of the low-frequency clock signal is smaller than 32.768 KHz (X<32768 Hz), time error (Terror) generated for 1 hour and a pulse count difference can be expressed in the following equation (4).                               T          eroor                =                              (                                          1                X                            -                              1                32768                                      )                    ×          32768          ×          3600                                    (        4        )                                                      T            eroor                                1            X                          =                  3600          ⁢                      (                          32768              -              X                        )                                              (        5        )            
As shown in equation (5), if the frequency (X) of the low-frequency clock signal is smaller than 32.768 KHz, as many as 3600 (32768−X) are not counted by the RTC circuit 10 for 1 hour, and time information outputted from the RTC circuit 10 is too low relative to actual time.
There are many problems with time generation in the related art. For example, as described above, if the frequency of the low-frequency clock signal generated from the crystal oscillator is changed due to an external condition (a temperature or a voltage), an error occurs in the time information (second, minute, hour) generated from the RTC circuit 10. This makes it impossible for the user to obtain accurate time information from the GSM or other terminal.
In order to compensate for the shortcomings of the related art, a method for implementing a crystal oscillator by using crystal with less error has been proposed. However, such a crystal is relatively expensive compared to the more rudimentary low-priced crystal. Moreover, it is difficult to obtain an accuracy of better than 3˜5 ppm (0.26˜0.43 sec/day) even with a more stable crystal.